This invention relates to a vibration type or oscillatory angular velocity sensor.
The angular velocity sensors (gyro sensors) are chiefly classified into a mechanical type utilizing the precession of a rotary body, an optical type utilizing a light-receiving timing change in accordance with the revolution of a laser beam rotating in a casing, a fluid type sensing a hot wire temperature representing an injection amount of a sensing gas changing in accordance with the rotation of a casing when the sensing gas is injected to a hot wire in the casing, and others.
On the other hand, the angular velocity sensors are recently required for detecting the traveling direction of an automotive vehicle in a car navigation system or the like. The vibration type angular velocity sensor is preferably employed in such systems because the vibration type is advantageous in cost and weight compared with the above-described other types. The vibration type angular velocity sensor has a vibrator oscillating in a predetermined reference direction and, when an angular velocity acts on this vibrator, detects a new oscillatory component based on a Coriolis force (hereinafter, referred to as an angular velocity oscillatory component) produced in a sensing direction perpendicular to the standard vibrating direction, and then outputs angular velocity information based on the detected oscillatory component. For example, according to a car navigation system, monitoring a present position based on a GPS (Global Positioning System) enables to sense a rough advancing direction of an automotive vehicle but cannot follow a sudden direction change of the vehicle at an intersection or the like. Thus, it is necessary to sense a turning motion of the vehicle based on the angular velocity. In this case, the turning directional angle is calculated by integrating momentary values of the angular velocity
However, the vibrator's motion components detected by the above-described vibration type angular velocity sensor are not always limited to the Coriolis force originating from the angular velocity. When this sensor is installed on an automotive vehicle or the like, the vibrator's motion components will include unnecessary acceleration components superposed on the Coriolis force component, such as sudden impact or any other vibrations caused by non-angular velocity factors. These unnecessary acceleration components appear as noises against the angular velocity to be sensed and accordingly deteriorate the sensing accuracy of the sensor when this sensor is used in detecting the traveling direction of the automotive vehicle.
The Japanese Patent Application Laid-open No. 2001-153659 discloses a conventional angular velocity sensor that includes two combined sensors vibrating in the standard vibrating direction with mutual opposite phases and outputs a finalized angular velocity waveform corresponding to a difference between the angular velocity oscillatory component waveforms of two sensors. When the vibrators oscillate with mutually opposite phases, the angular velocity oscillatory components detected by these vibrators are also in the mutually opposite-phase relationship. On the other hand, the above-described unnecessary acceleration components appear as in-phase components. Hence, obtaining a differential waveform can cancel these in-phase acceleration components and leave only the necessary angular velocity oscillatory components. The sensing accuracy can be increased.
However, besides the vibrator (e.g. detection weight), the vibration type acceleration sensor generally includes a vibrator driving section for actuating the vibrator (for example, comb-teeth electrodes and piezoelectric elements disclosed in the above-described prior art document) and a detecting section detecting an angular velocity oscillatory component (for example, a capacitance type displacement sensing section). These components are inevitably subjected to manufacturing errors that possibly cause sensor individual differences in their angular velocity sensing properties. The sensor individual differences cause a difference in the sensing accuracy of the in-phase acceleration components that may be detected by two combined sensors causing the above-described opposite-phase vibrations. Accordingly, some of the in-phase components will reside even if the above-described processing for obtaining the differential waveform is carried out. The angular velocity sensing accuracy will thus deteriorate.
The above-described prior art document discloses a capacitance type displacement sensing section, more specifically a vibration detecting capacitor capable of changing the distance between electrodes in response to an applied angular velocity oscillatory component. A constant bias voltage is applied to this vibration detecting capacitor. A charge amount change of the vibration detecting capacitor occurring in accordance with the change of the distance between electrodes is converted into a voltage waveform and produced as an angular velocity sensing waveform. The factors causing the difference in sensor's acceleration sensing accuracy include variation or fluctuation of the bias voltage applied to the vibration detecting capacitor. To eliminate this, the above-described prior art document proposes utilizing an imaginary short-circuit of an operational amplifier in producing a differential waveform from outputs of a plurality of sensors and supplies the voltage of a reference power source to the vibration detecting capacitors of respective sensors. According to this system, it will be possible to reduce the adverse influence given by the variation or fluctuation of the bias voltage. However, this system is not effective at all in reducing adverse influences given by the variation factors other than the bias voltage, such as the weight and size of the vibrator (e.g. detection weight), the specifications of the vibrator driving section, or the electrode area of the vibration detecting capacitor.